Method of biaxially aligning crystalline material

ABSTRACT

A method of inducing biaxial particle alignment in a body of crystalline particles of a material such as a high temperature superconducting material, having anisotropic magnetic susceptibility so that at least a major portion of the crystalline particles have at least two crystalline axes generally parallel, comprises subjecting the particles to a magnetic field which varies cyclically relative to the body of crystalline particles with time, and which has an average magnitude which is a maximum in a first direction, a lower average magnitude in a second direction generally orthogonal to said first direction and a minimum or zero average magnitude in a third direction generally orthogonal to said first and second directions. Alignment of the axis of maximum magnetic susceptibility of the particles generally with the direction of maximum average magnitude and the axis of minimum magnetic susceptibility of the particles generally with the minimum or zero average magnitude direction is induced.

FIELD OF INVENTION

The invention comprises a method of inducting biaxial particulatealignment in a body of crystalline particles possessing anisotropicmagnetic susceptibility, such as high temperature superconductingmaterial.

BACKGROUND

Many High-T_(c) Superconducting Cuprates (HTSC) are known to havesuperconducting transition temperatures, T_(c) exceeding the temperatureat which liquid nitrogen boil, 77K. As such they have a potentiallylarge number of applications ranging from power generation,distribution, transformation and control, to high-field magnets, motors,body scanners, telecommunication and electronics. A high T_(c) valuealone does not guarantee the utility of these HTSC at 77K or highertemperatures. Often these applications require large critical currentsin the HTSC and this is not achieved unless the crystalline grains orparticles of the HTSC are crystallographically aligned. This is commonlyachieved in thin-films wherein the HTSC material is deposited on asubstrate in such as way as to obtain crystallographic alignment of thematerial. However, thin film, while supporting very high criticalcurrent densities, J_(c) do not carry a very high absolute criticalcurrent, I_(c) because they are so thin.

Superconducting wires or other components which use bulk superconductingmaterial can in principle support much higher I_(c) values provided theycan be textured to achieve high J_(c) values. In processing HTSC to forsuch wires or similar, aligning the crystalline particles of the HTSC sothat a major portion or ore of the particles have at least one similaraxis parallel such as the c-axis, is commonly referred to as texturingthe material. It has become apparent, at least for some and probably allHTSC, that crystallographic alignment along one common axis or monoaxialtexture, may be insufficient to achieve high critical current densityand the full biaxial texture in which two similar axes of thecrystalline particles, such as the c- and b-axes, are aligned ispreferable. An example of monoaxial texture of HTSC achieved by magneticmeans is given by Tkaczyk and Lay (J. Mater. Res. 5 (1990) 1368) inwhich no significant improvement over unaligned material was seen.

It is known that such biaxial texturing or alignment can be achieved bylinear melt processing in which the HTSC material is pulled slowlythrough a temperature gradient so that part of the material residesabove the partial melting point and another part lies below, and themelt/solid interface is slowly displaced along the length of thematerial leaving behind dense, textured material in its path. Thisprocess, however, is difficult to control and extremely slow, producingbiaxially textured material at a rate as low as 1 mm/hour. Linear meltprocessing is considered unsuitable for manufacturing long length wiresexceeding 100 m or, worse still, 1 km.

Efforts have also been made to produce biaxial texture by a combinationof monoaxial magnetic alignment and mechanical treatments such aspressing or rolling (Chen et al, Appl. Phys. Lett. 58 (1991) 531).However, the difficulty of achieving bulk alignment by mechanical meanshas prevented significant gains in critical current from beingdemonstrated.

SUMMARY OF INVENTION

The invention provides an improved or at least alternative method ofinducing biaxial particulate alignment in a body of crystallineparticles processing anisotropic magnetic susceptibility, such as hightemperature superconducting material, which is useful as a step informing bulk HTSC components such as wires, tapes, or other conductingelements, or other components.

In broad terms the invention comprises a method inducing biaxialparticulate alignment in a body of crystalline particles havinganisotropic magnetic susceptibility, so that at least a major portion ofthe crystalline particles have at least two crystalline axes generallyparallel, comprising subjecting the particles to a magnetic field whichvaries cyclically relative to the body of crystalline particles withtime and which has an average magnitude which is a maximum in a firstdirection, lower in a second generally orthogonal direction, and aminimum or zero in a third direction generally orthogonal to the firstand second directions, to induce alignment of the axis of maximummagnetic susceptibility of the particles with the field direction ofmaximum average magnitude and the axis of minimum magneticsusceptibility with the field direction of minimum or zero averagemagnitude.

In one for of the invention a time varying magnetic field relative tothe crystalline particles is produced by causing cycles of relativerotation between the body of crystalline particles and a magnetic fieldwith the crystalline particles being subjected to the magnetic field inone orientation between the body of crystalline particles and themagnetic field for a time longer than the crystalline particles aresubjected to the magnetic field in a second such orientation. The bodyof crystalline particles may be rotated relative to a magnetic field orthe magnetic field may be rotated relative to the crystalline particles.

The relative may be cyclical between two positions with, in each cycle,the time at one position being less than the time at the other position.The angle of relative rotation may be between 0° and 180° and ispreferably about 90°. Alternatively the relative rotation may becyclical between three positions. The body of particles may be rotatedwithin a magnetic field, between one position in which the body ofparticles spends most time so that the direction of the magnetic fieldrelative to the particles in this position defines the direction ofmaximum average field magnitude relative to the particles, and twopositions on either side of the maximum magnitude position, in which theparticles spend less time. In these positions the angle of the fieldrelative to the maximum average magnitude field direction may be between0° and 180° and is preferably about 90°. For example biaxial alignmentmay be induced by cyclically rotating a body of particles about an axisfirst by 90° with respect to an initial orientation in which the body ofparticles spends most time. The body of particles is then returned tothe initial position and then rotated by 90° in the opposite direction,and is then returned to the initial position and the cycle is repeated.The total time spend at the opposite positons (±90°) is less than thetime spent at the initial position so that the field direction when thebody of particles is in the initial position defines the direction ofthe maximum average field magnitude relative to the particles. Theaverage field magnitude along an axis parallel to the +90° and −90°positions is lower and the crystalline axis of intermediate magneticsusceptibility of the particles will tend to align along tis axis. Thesign reversal of the magnetic field at opposite positions is unimportantfor the aligning process as the potential energy of a body in a magneticfield of strength B varies as B². Initial alignment may be accomplishedby holding the body of crystalline particles in the maximum averagemagnitude position to induce an initial alignment of the crystals withtheir axis of maximum magnetic susceptibility along this fielddirection.

In another form of the invention the time varying magnetic field may bea net magnetic field which is the sum of a field in one direction and afield in a second direction, the strength of which field in the seconddirection varies with time. The direction of the first field relative tothe crystalline particles (when the second field is switched off or at aminimum) will generally be the field direction of maximum averagemagnitude and the crystalline axis of maximum magnetic susceptibilitywill align in this direction. When the second field is switched on or ata maximum, it adds to the field in the first direction and the directionorthogonal to both the first and second fields is the direction ofminimum average magnetic field magnitude, with which the axis of minimummagnetic susceptibility of the particles will align. Alternatively thefield in one direction may be switched on while the field in anotherdirection is switched off and vice versa. The field in one direction maybe stronger than the field in the other direction or may be on a for alonger time in each cycle.

In another form of the invention a time varying magnetic field relativeto the crystalline particles is produced by causing relativetranslational movement between the body of crystalline particles and aseries of electromagnets or permanent magnets oriented alternately inone and the another direction. Either the crystalline particles may bemoved relative to the series of magnets or a series of longitudinallyarranged magnets may be moved relative to the crystalline particles.Either the body of particles may be moved more slowly past or stoppedadjacent each alternate magnet, or alternate magnets may be loner in thedirection of movement, or alternate magnets may be of higher strength.The net effect is that the particles are subjected to a magnetic fieldthe direction of which varies with time relative to the particles andwhich has average magnitude which is a maximum in one direction, lowerin a second direction, and zero or a minimum in a third direction, toachieve a common biaxial alignment of the crystalline particles. Insteadof a series of magnets, a similar effect may be achieved by appropriateshaping of a magnetic core or by varying the density of windings of anelectromagnet along the length of a core, to form a series of magneticsubfields.

The magnetic alignment may be carried out while the crystallineparticles are settled through or suspended in an organic liquid, epoxy,polymer solution, molten wax, or other fluid, or a gaseous or liquidflow so that they are free to move under the influence of the appliedmagnetic fields. Where the crystalline particles are settled through afluid they will form a sediment of aligned particles. Alternatively thesuspension may have the property that, after alignment has beenachieved, it can be arrested to preserve the texture, by solidification,curing, polymerisation, or chemical reaction, for example. Alternativelythe fluid may be slowly drained off, or partially or completelyvolatilised or evaporated, or a suspending gaseous or liquid flow may beslowly ceased. Alternatively again the body of crystalline particles maybe vibrated while subjected to the magnetic alignment, or the magneticalignment may be carried out during electrophoretic deposition.

In a preferred form of the invention as applied to rare earth HTSCsuperconductor the particles used for alignment will be R₂Ba₄Cu₇O_(15-δ)(where R is Y or a rare earth element) or RBa₂Cu_(4O) ₈ as theseparticles may be conveniently prepared as detwinned in contrast toRBa₂Cu₃O_(7-δ) which is typically twinned.

The magnetic alignment of the invention may be carried out using anelectromagnet or electromagnets, a permanent magnet or magnets togenerate the magnetic field, or using radio frequency or microwave meansfor example to generate electromagnetic fields.

The magnetic alignment may be carried out on crystalline materialcrystallising from a melt to induce biaxial texture in the resultingsolid, by applying a time varying magnetic field to the crystallisingmaterial, and the term “particles” is to be understood accordingly, asincluding crystal nuclei for example.

DESCRIPTION OF FIGURES

The invention is further described with reference to the accompanyingfigures, by way of example and without intending to be limiting. In thefigures:

FIG. 1 illustrates inducing biaxial alignment by one preferred form ofthe method of the invention,

FIGS. 2a and 2 b illustrate inducing biaxial alignment by anotherpreferred form of the method of the invention,

FIG. 3 illustrates inducing biaxial alignment b a further preferred formof the method of the invention,

FIG. 4 illustrates inducing biaxial alignment b a further preferred formof the method of the invention,

FIGS. 5a, 5 b, and 5 c and 5 d show XRD traces measured on a sample ofDy₂Ba₄Cu₇O_(15-δ) biaxially textured as described in Example 1—L showsthe strong (00l) lines for the sample face perpendicular to the fielddirection, T denotes the trace for the surface normal to the horizontaltransverse direction and Z for the surface normal to the verticaltransverse direction. Differences between these traces reveal biaxialtexture,

FIG. 5d shows the rocking curve for the (0 2 0) diffraction line of aY₃Ba₄Cu₇O_(15-δ) sample aligned as described in Example 1, and

FIG. 6 shows XRD traces measured on a sample of Y₃Ba₄Cu₇O_(15-δ)biaxially textured as described in example 2—L, Z and T indicate as inFIGS. 5a to 5 c.

DESCRIPTION OF PREFERRED FORMS

In the following the crystal axis of maximum magnetic susceptibility isreferred to as the M1 axis, the intermediate susceptibility axis as theM2 axis, and the axis of minimum magnetic susceptibility as the M3 axis.The maximum average magnitude magnetic field direction with which thecrystal axis M1 will tend to align is denoted B1 and the lower averagemagnitude field direction along which axis M2 will tend to align isdenoted B2.

Referring to FIG. 1, in one form an iron-cored electromagnet 1, or anassembly of permanent or trapped-flux magnets or other means providing amagnetic field, is rotated about an axis 2 through the centre of thepole gap as shown, in which a suspension of HTSC particle is positionedor through which the suspension passes. The direction of the M1alignment is determined b the direction of maximum average magneticfield magnitude. The magnet 1 may be initially held at this orientationto induce an initial uniaxial alignment. To induce a biaxial alignmentthe magnet is cyclically rotated, as indicated by arrow A, alternatelyto opposite positions through similar angles of greater than 0°, lessthan 180° and such as about 90°, with respect to the 0° position forshort periods which are also short compared to typical grain orientationtimes. Rotation between 0° and only one other position such a the +90°position and back may be used. Instead of rotating the magnet relativeto the HTSC particles, the body of HTSC particles may be rotatedrelative to the magnet.

Referring to FIGS. 2a and 2 b, in another form a suspension of HTSCparticles is positioned in or passed axially through the bore of anair-cored solenoidal electromagnet 3 which is situated between he polesof an iron-cored electromagnet 4. The axis of the solenoid 3 is directedperpendicular to the field generated by the iron-cored magnet 2 asshown. The field direction of the iron-cored magnet 2 extending indirection B1 has the maximum average magnitude and imparts a strongtexture to the HTSC particles along crystal axis M1. The solenoid 2 ispulsed with electric current to generate a pulsed or oscillating fieldin direction B2, with the pulse length short compared with the averagegrain alignment time, to also align the M2 axis of the crystals alongaxis B2. The net time varying field to which the crystals are subjectedis the sum of the fields from the solenoid 3 and electromagnet 4.

Referring to FIG. 3, in another form both the initial aligning field B1and a pulsed field B2 are produced by solenoids or coils 5 and 6. The M1crystal axis aligns with the field B1 of coil 5 which has higher averagemagnitude, while coil 6 is pulsed with current to produce a pulsed oroscillating field in direction B2 of lower average magnitude so that theM2 axis of the HTSC crystals aligns along axis B2. Alternatively thesolenoid 5 may also be pulsed to produce a stronger pulsed field thancoil 6, or a field of similar strength but with a longer duty cycle soas to have a higher average field strength. Again the time varying fieldto which the crystals are subjected is the sum of the fields from thesolenoids 5 and 6.

Referring to FIG. 4, in another form a suspension of the HTSC is movedcontinuously or stepwise past a series of magnets alternately with fielddirections B1 and B2. The time spent in the B1 and B2 field regionsand/or the relative strengths of the field in the B1 and B2 directionsare controlled so that the average field strength in the B2 direction isless than the average field strength in the B1 direction, so thatcrystal axis M1 aligns with field axis B1 and crystal axis M2 alignswith field axis B2. A similar series of magnetic fields may also beproduced by a succession of asymmetrically positioned core segments in asolenoid for example. The net field to which the HTSC is subjected asthe HTSC moves past the magnets is a magnetic field the direction ofwhich varies with time with an average magnitude which is a maximum inone direction lower in an orthogonal direction, and zero or minimum in athird direction orthogonal to the tow other directions, which will againinduce biaxial magnetic alignment.

In a particularly preferred form of the invention a powdered HTSC suchas R₂Ba₂Cu₃O_(7-δ) or R₂Ba₄Cu₇O_(15-δ), (where R is for Y or a rareearth element) may be dispersed with the aid of a suitable surfactant ina volatile fluid such as heptane, which is introduced to the inside of acontainer such as a tube or trough of silver of sintered HTSC, or ofceramic with or without protective layers of materials selected forlimited reactivity with the powdered HTSC. The tube or similarcontaining the dispersed HTSC may then be subjected to biaxial magneticalignment according to the method of the invention. The tube isventilated so that the suspension is able to dry after alignment hasbeen achieved. In the case of Y₂Ba₄Cu₇O_(15-δ), a magnetic texture maybe produced in which the b-axis is aligned along the tube length and thec-axis across the tube length, for example.

In another particularly preferred form of the invention a powdered HTSCmay be suspended, optionally with the aid of a suitable surfactant, in avolatile fluid which is coated on the exterior of a substrate componentsuch as metal tape which is then passed continuously through a region orzone in which the tape carrying the fluid suspending the HTSC particlesis subjected to biaxial magnetic alignment by a time varying magneticfield according to the method of the invention, to biaxially texture theHTSC particles. The solvent may be evaporated to fix the aligned HTSCparticles to the tape while maintaining their alignment, or may sedimentthrough the thin solvent layer on the tape, or may be deposited on thetape by electrophoresis.

The following examples further illustrate the invention:

EXAMPLE 1

Biaxial alignment of Dy₂Ba₄Cu₇O_(15-δ)(Dy-247) was achieved usingapplied magnetic fields alone. FIG. 5a shows x-ray diffractometer tracesmeasured on a sample of Dy-247 cured in epoxy while being rotated in ahorizontal 1 T field. After being held several minutes stationary in thefield the mould containing the sample was rotated successively about avertical axis to orientations rotation −90°, 0°, +90° and 0° withrespect to its original orientation. The dwell time at 0° was of order 2seconds, at the transverse orientations half this time. The trace inFIG. 5a labelled L shows the trace obtained on the sample faceperpendicular to the field direction; the strong (ool) lines indicatec-axis texture. The traces labelled T and Z are for the surfacesrespectively normal to the horizontal transverse direction and thevertical transverse direction. Any difference in these traces reveals adegree of biaxial alignment. Trace T is dominated by the (200) line,indicating a strong a-axis texture. Trace Z is dominated by the (020)line, indicating a strong b-axis texture. FIG. 5b shows these lines ingreater detail and FIG. 5c shows the corresponding traces from astationary sample which shows only uniaxial alignment. The biaxialtexture obtained indicates that the magnitude of the susceptibilityalong the respective crystal axes reduces in the order CAB. Byexperimentation the following R₂Ba₄Cu₇O_(15-δ) compositions have beenidentified as CAB biaxial aligners: R=Y, Dy, Nd, as BAC biaxialaligners; R=Er, Eu, Yb; and as an ACB aligner: R=Gd. The precise grainorientation attainable using biaxial magnetic alignment is demonstratedby the rocking curve for the (0 2 0) diffraction line of a Y-247 samplealigned in epoxy. Shown in FIG. 5d. In this case the b-axis texture hasa full width at half maximum of just over 2°.

EXAMPLE 2

Biaxial alignment of Y-247 grains in epoxy was achieved using a rotatingfield apparatus with the sample remaining stationary. A magnetic fieldof 0.85 T was produced in a 10 mm gap using pairs of NdFeB permanentmagnets mounted in a steel yoke rotatable about a horizontal axis. Thefield was alternated between horizontal and vertical orientations atfrequencies around 0.5 Hz. The ratio of time spent in the major field(L) orientation to time spent in the perpendicular (T) orientation wastypically 2:1. Field alternation was continued for several hours whilethe epoxy set. FIG. 6 shows XRD traces recorded on mutuallyperpendicular surfaces with the L, Z, and T orientations as in Example1, revealing a similar biaxial texture.

The invention has been generally described with reference to the biaxialalignment of HTSC material but may also be used to induce biaxialcrystalline alignment in other crystalline materials having anisotropicmagnetic susceptibility.

The foregoing descries the invention including preferred forms thereof.Alterations and modifications as will be obvious to those skilled in theart are intended to be incorporated within the scope hereof, as definedin the accompanying claims.

What is claimed is:
 1. A method of inducing biaxial particle alignmentin a body of crystalline particles having anisotropic magneticsusceptibility, so that at least a major portion of the crystallineparticles have at least two crystalline axes generally parallel,comprising subjecting the particles to cycles of relative rotationbetween the body of crystalline particles and a magnetic field in afirst direction relative to the body of crystalline particles, and amagnetic field in a second direction relative to the body of crystallineparticles, which field in said second direction has a lower magnitude,or lower average magnitude over time, than the magnitude or averagemagnitude over time of said magnetic field in said first direction, saidcrystalline particles having freedom to move to align with said magneticfield, to thereby induce alignment of the axis of maximum magneticsusceptibility of the particles generally with said field in said firstdirection and an axis of lower magnetic susceptibility of the particlesgenerally with said field in said second direction, and the crystallineparticles being subjected to the magnetic field in said first directionin one orientation between the body of crystalline particles and themagnetic field, and for a time longer than the crystalline particles aresubjected to the magnetic field in said second direction, in anothersuch orientation.
 2. A method according to claim 1, comprising causingsaid relative rotation by cyclically rotating said magnetic fieldrelative to the body of crystalline particles.
 3. A method according toclaim 1, comprising causing said relative motion by cyclically rotatingsaid body of crystalline particles relative to said magnetic field.
 4. Amethod according to claim 1, wherein said first direction and saidsecond direction are substantially perpendicular to one another.
 5. Amethod according to claim 4, wherein the strength of the field in saidfirst direction and/or second direction also varies with time.
 6. Amethod according to claim 1 including causing relative transitionalmovement between the body of crystalline particles and a series ofmagnets oriented alternately in said first and then said seconddirections.
 7. A method according to claim 6, wherein the body ofcrystalline particles is moved relative to said series of magnets.
 8. Amethod according to either one of claim 6 or 7, wherein said firstdirection and said second direction are substantially perpendicular toone another.
 9. A method of inducing biaxial particle alignment in abody of crystalline particles having anisotropic magneticsusceptibility, so that at least a major portion of the crystallineparticles have at least two crystalline axes generally parallel,comprising the steps of subjecting the particles to cycles of: amagnetic field in a first direction relative to the body of crystallineparticles, and a magnetic field in a second direction relative to thebody of crystalline particles, which field in said second direction hasa lower magnitude, or lower average magnitude over time, than themagnitude or average magnitude over time of said magnetic field in saidfirst direction, said crystalline particles having freedom to move toalign with said magnetic field, to thereby induce alignment of the axisof maximum magnetic susceptibility of the particles generally with saidfield in said first direction and an axis of lower magneticsusceptibility of the particles generally with said field in said seconddirection; and creating and varying said field in said first directionand said field in said second direction by pulsing each of anelectromagnet oriented in said first direction and an electromagnetoriented in said second direction relative to the body of crystallineparticles.
 10. A method of inducing biaxial particle alignment in a bodyof crystalline particles having anisotropic magnetic susceptibility, sothat at least a major portion of the crystalline particles have at leasttwo crystalline axes generally parallel, comprising subjecting theparticles to repeated alternating cycles between a magnetic field in afirst direction relative to the body of crystalline particles and amagnetic field in a second direction relative to the body of crystallineparticles and which is generally orthogonal to said first direction,with the strength of said magnetic field in said first direction and thestrength of said magnetic field in said second direction rleative to oneanother, or the time for which the particles are subjected to saidmagnetic field in said first direction relative to the time for whichthe particles are subjected to said magnetic field in said seconddirection, being such that over said repeated alternating cycles betweensaid magnetic field in said first direction and said magnetic field insaid second direction, said magnetic field in said second direction hasa lower average magnitude over time that the average magnitude over timeof said magnetic field in said first direction, said crystallineparticles have freedom to move to align with said magnetic field, tothereby induce alignment of said axis of maximum magnetic susceptibilityof at least a major portion of the particles generally with said fieldin said first direction and an axis of lower magnetic susceptiblity ofthe particles generally with said field in said second direction.
 11. Amethod according to claim 10, comprising subjecting the particles tosaid repeated alternating cycles by causing cycles of relative rotationbetween the body of crystalline particles and a magnetic field so thatthe crystalline particles are subjected to the magnetic field in saidfirst direction in one orientation between the body of crystallineparticles and the magnetic field, and to the magnetic field in saidsecond direction in another orientation between the body of crystallineparticles and the magnetic field, and such that the body of crystallineparticles is subjected to the magnetic field in said first direction fora time longer than the crystalline particles are subjected to themagnetic field in said second direction in each cycle.
 12. A methodaccording to claim 11, comprising causing said relative rotation bycyclically rotating means providing said magnetic field relative to thebody of crystalline particles.
 13. A method according to claim 11,comprising causing said relative rotation by cyclically rotating saidbody of crystalline particles relative to said magnetic field.
 14. Amethod according to claim 10 comprising subjecting the particles to saidrepeated alternating cycles by creating said magnetic field in saidfirst direction with a first electromagnet and said magnetic field insaid second direction with a second electromagnet and pulsing saidelectromagnets alternately, and pulsing said first electromagnet for atime longer than said second electromagnet in each cycle or pulsing saidfirst electromagnet so as to generate a magnetic field of higherstrength than the magnetic field generated by said second electromagnetin each cycle.
 15. A method according to claim 10 comprising subjectingthe particles to said repeated alternating cycles by causing relativetranslational movement between the body of crystalline particles and aseries of magnets oriented alternatively in said first and then saidsecond directions.
 16. A method according to claim 15, wherein the bodyof crystalline particles is moved relative to said series of magnets.